StarChild Question of the Month for December 2002

Question:

Why is space black?

Answer:

Your question, which seems simple, is actually very difficult to answer!
It is a question that many scientists pondered for many centuries -
including Johannes
Kepler, Edmond
Halley , and German physician-astronomer Wilhelm Olbers.

There are two things to think about here. Let's take the easy one
first and ask "why is the daytime sky blue here on Earth?" That is a
question we can answer. The daytime sky is blue because light from the nearby
Sun hits molecules in the Earth's atmosphere and scatters off in all
directions. The blue color of the sky is a result of this scattering
process. At night, when that part of Earth is facing away from the Sun,
space looks black because there is no nearby bright source of light,
like the Sun, to be scattered. If you were on the Moon, which has no
atmosphere, the sky would be black both night and day. You can see this in
photographs taken during the Apollo Moon landings.

So, now on to the harder part - if the universe is full of stars,
why doesn't the light from all of them add up to make the whole sky
bright all the time? It turns out that if the universe was infinitely
large and infinitely old, then we would expect the night sky to be
bright from the light of all those stars. Every direction you looked
in space you would be looking at a star. Yet we know from experience
that space is black! This paradox is known as Olbers' Paradox. It is a paradox because of the apparent contradiction between our expectation that the night
sky be bright and our experience that it is black.

Many different explanations have been put forward to
resolve Olbers' Paradox. The best solution at present is that the
universe is not infinitely old; it is somewhere around 15 billion years
old. That means we can only see objects as far away as the distance
light can travel in 15 billion years. The light from stars farther away
than that has not yet had time to reach us and so can't contribute to
making the sky bright.

Another reason that the sky may not be bright with the visible light of
all the stars is because when a source of light is moving away from you,
the wavelength of that light is made longer (which for light means more
red.) This means that the light from stars that are moving away from us
will become shifted towards red, and may shift so far that it is
no longer visible at all. (Note: You hear the same effect when an
ambulance passes you, and the pitch of the siren gets lower as the
ambulance travels away from you; this effect is called the
Doppler Effect).